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Watching the clown's antics, she looked like she was about to burst out laughing.
Literal
She [topic-は] clown [possessive-の] gestures [object-を] watching, any-moment-now laugh-burst-out seemed-about-to.
Packs three grammar layers into one predicate: 笑い出す (compound verb with ~出す marking sudden onset — 'burst out laughing'), ~そう (appearance-based inference — 'looks like she's about to'), and ~だった (past tense — this was the state at that moment). 今にも intensifies the imminence: 'at any moment now.' 道化師 is a somewhat literary word for 'clown' — ピエロ (from French) is more common in casual speech. 仕草 covers gestures, mannerisms, and body language.